1. Field of the Invention
The present invention relates to a discharge ionization current detector to be used as a detector of an analysis device such as a gas chromatograph.
2. Description of the Related Art
As a detector capable of detecting, at high sensitivity, from inorganic substances to low boiling point organic compounds, there is a pulsed discharge ionization current detector (PDD). The discharge ionization current detector excites plasma generating gas such as the molecules of helium by high-voltage pulsed discharge, and ionizes a molecule to be analyzed by using optical energy generated at the time of the molecules returning from the excited state to the ground state. Then, a detection signal according to the amount of the molecule to be analyzed is obtained by detecting the ion current (see JPA 2010-060354).
Such a conventional discharge ionization current detector includes a plasma generating unit at its upper part, and a sample ionization/detection unit at its lower part. The plasma generating unit is provided as one section of a dielectric tube, such as a quartz tube, for example, through which the plasma generating gas, such as helium, flows. Three, upper, middle and lower, ring-shaped electrodes are attached to the outer circumference of the dielectric tube so as to encircle the outer circumference in the flow direction of the plasma generating gas. The part between the upper electrode and the lower electrode is the plasma generating unit. A high alternating voltage is applied to the middle electrode by a high voltage AC source, and the upper electrode and the lower electrode are grounded. The plasma generating gas is excited by dielectric barrier discharge occurring between the upper, middle and lower electrodes.
The sample ionization/detection unit includes a charging electrode and an ion current detecting electrode. A sample gas supply tube for supplying sample gas to the sample ionization/detection unit is connected to the sample ionization/detection unit on the other side from the plasma generating unit. A plasma generating gas injection port of the dielectric tube of the plasma generating unit and a sample gas injection port of the sample gas supply tube are arranged facing each other, and a gas exhaust port for exhausting gas to outside is provided therebetween.
With such a discharge ionization current detector, almost all of the amount of sample ion generated by the sample ionization unit is affected by the amount of excitation light from the plasma generating unit, and the amount of excitation light from the plasma generating unit is affected by the discharge state of the plasma generating unit. That is, when the discharge state of the plasma generating unit is changed, the amount of excitation light from the plasma generating unit is changed, thereby affecting the amount of sample ions generated by the sample ionization unit. Therefore, it is important to stabilize the discharge state of the plasma generating unit.
As one factor preventing stabilization of the discharge state of the plasma generating unit, entering of sample gas into the plasma generating unit can be cited. With a discharge ionization current detector as described above, when the amount of supply of sample gas from the sample gas supply tube increases, the sample gas sometimes enters the dielectric tube of the plasma generating unit and reaches the plasma generating unit. When the sample gas reaches the plasma generating unit, discharge becomes less likely to occur in the plasma generating unit, causing problems such as reduction in the detection sensitivity due to weakening of plasma intensity, or in the worst case, the plasma itself is extinguished and measurement itself is made impossible.